Thermotropic, fully aromatic polyesters based on p moles of p-hydroxybenzoic acid, q moles of terephthalic acid or isophthalic acid and r moles of diphenols, such as hydroquinone or 4,4'-dihydroxydiphenyl, are known from West German Laid-open Application 2,205,971. The conditions of q:r=10:15 to 15:10, p:q=1:100 to 100:1 and p+q+r=3 to 600 are valid in this application. These polyesters can be readily processed only if terephthalic acid is largely replaced by the crystallinity-disturbing isophthalic acid, which leads, however, to a not inconsiderable deterioration of some important mechanical and thermal properties.
According to European Pat. No. 134,956, thermotropic, fully aromatic polyesters on the basis of (a) p-hydroxybenzoic acid, (b) iso- and if necessary terephthalic acid, (c) hydroquinone and (d) 3,4'- and/or 4,4'-dihydroxydiphenyl, 3,4'- and/or 4,4'-dihydroxydiphenyl ether and/or 3,4'- and/or 4,4'-dihydroxydiphenyl sulfide are known, the molar ratio of the condensed groups a:b being equal to 80:20 to 50:50, the molar ratio of the condensed groups b:c:d being equal to 20:10:10 to 50:48:2, the molar ratio of the condensed groups b:(c+d) being equal to 0.95 to 1.05 and the terephthalic acid proportion of component (b) amounting to as much as 50 mole %. However, the Vicat B temperatures of the described mixed polyesters lie at the relatively low level of 128.degree. to 145.degree. C. and their usability is limited.
An object of the present invention is to provide thermotropic, fully aromatic polyesters of the highest possible high-temperature shape stability, which can be thermoplastically processed below 400.degree. C. to shaped articles with excellent mechanical properties.